Fluid couplings of the kind here considered, including a motor-driven impeller and a load-connected turbine defining between them a working chamber filled with hydraulic liquid, are often used underground in cases requiring large driving powers in the face of limited network energy feeding the various electric motors involved. These motors are generally of the induction or asynchronous type which have an extended startup period along with a considerable slip range. Such an induction motor, e.g. when connected with a drive shaft of a sprocket operating a chain conveyor alongside a mine face, may be difficult to restart after prolonged standstill, as when rocks dropping onto the conveyor belt include wet coal jamming that belt. In that event the conveyor often must be unloaded by hand before being again set in motion for the delivery of further mined material. It therefore becomes important to optimize the torque-transmission ratio of each fluid coupling for the purpose of minimizing or eliminating the time of production slowdown during which miners working at the conveyor are exposed to the hazard of coal or overburden falling from the mine face. The solution of coupling a pair of such motors with opposite ends of the sprocket shaft is not always sufficient.
In other instances, as where the load being driven by an inducation motor via a fluid coupling is a conveyor with a highly elastic band of rubber of the like, the conveyor itself must be started at a low speed which is progressively increased.
it has already become the practice to provide fluid couplings with an antechamber from which hydraulic liquid can be delivered at a controlled rate, e.g. by means of scoop-type feeders, to the working chamber for varying the torque-transmission ratio between the impeller and the turbine. These assemblies, however, are rather bulky and not practical in many instances, particularly at underground sites in the vicinity of a mine face.